NEW YORK, Aug 29 (Reuters) - Energy investors have taken bets for years on what they thought was an important indicator of future energy production: the weekly rig count data provided by oil service firms.They may want to be careful about how much money they put on the table.

A Reuters analysis of the data, and interviews with officials at companies involved in collecting and compiling it, shows that it may sometimes be an arbitrary and misleading gauge subject to revisions.

Thousands of rigs currently drilling in Texas, home to a portion of the Granite Wash and the Eagle Ford shale formation - another liquids-rich play that producers have flocked to over the past year - changed classification between permitting and when production was officially logged months later based on actual well data, according to state records.

Classifications do not necessarily depend on the contents of a well, but on what is driving the economics. A rig producing, on an energy basis, 50 percent natural gas, 20 percent natural gas liquids and 30 percent oil, can still be considered an oil well if that is the major revenue driver, Baker Hughes said.

The number of rigs may now be less telling than where they are located. Efficiencies innew oil and gas fields vary widely depending on the basin in which they are operating.

Gas produced from one rig in the Haynesville shale play in Louisiana and Texas produces about as much gas as 50 wells in the Permian, according to Adam Bedard, formerly at Bentek Energy in Colorado, which closely monitors production figures.

"There is an illusion that production is going to fall this year, an illusion driven by the declining rig count," he said.